Light olefins have traditionally been produced through the process of steam or catalytic cracking. Because of the limited availability and high cost of petroleum sources, the cost of producing light olefins from such petroleum sources has been steadily increasing. Light olefins serve as feeds for the production of numerous chemicals. As the emerging economies of the Third World strain toward growth and expansion, the demand for light olefins will increase dramatically.
The search for alternative materials for light olefin production has led to the use of oxygenates such as alcohols and, more particularly, to the use of methanol, ethanol, and higher alcohols or their derivatives. These alcohols may be produced by fermentation or from synthesis gas. Synthesis gas can be produced from natural gas, petroleum liquids, and carbonaceous materials including coal, recycled plastics, municipal wastes, or any organic material. Thus, alcohol and alcohol derivatives may provide non-petroleum based routes for the production of olefin and other related hydrocarbons.
Molecular sieves such as the microporous crystalline zeolite and non-zeolitic catalysts, particularly silicoaluminophospbates (SAPO), are known to promote the conversion of oxygenates to hydrocarbon mixtures. Numerous patents describe this process for various types of these catalysts: U.S. Pat. Nos. 3,928,483; 4,025,575; 4,252,479; 4,496,786; 4,547,616; 4,677,243; 4,843,183; 4,499,314; 4,447,669; 5,095,163; 5,191,141; 5,126,308; 4,973,792; and 4,861,938.
The process may be generally conducted in the presence of one or more diluents which may be present in the oxygenate feed in an amount between about 1 and about 99 molar percent, based on the total number of moles of all feed and diluent components fed to the reaction zone (or catalyst). Diluents include--but are not limited to--helium, argon, nitrogen, carbon monoxide, carbon dioxide, hydrogen, water, paraffins, hydrocarbons (such as methane and the like), aromatic compounds, or mixtures thereof. U.S. Pat Nos. 4,861,938 and 4,677,242 particularly emphasize the use of a diluent combined with the feed to the reaction zone to maintain sufficient catalyst selectivity toward the production of light olefin products, particularly ethylene. The above U.S. patents are hereby incorporated by reference.
U.S. Pat. No. 5,026,935 discloses a process for the preparation of ethylene from C.sub.4 or higher feed by the combination of cracking and metathesis to form ethylene and propylene and at least a portion of the propylene is metathesized to ethylene. U.S. Pat. No. 4,590,174 discloses an olefin metathesis process employing a catalyst comprising an inorganic refractory oxide support containing at least one of tungsten oxide and molybdenum oxide and a promoting agent for the disproportionation reaction. Example 3 of U.S. Pat. No. 3,723,562 describes the conversion of propylene to a mixture of ethylene and butenes using a WO.sub.3 -SiO.sub.2 catalyst containing about 8 weight percent tungsten oxide. Conditions used were 800.degree. F., 100 psig and 15 hr.sup.-1 WHSV. Propylene conversion was 19 percent with 37.6 percent selectivity to ethylene and 62.4 percent selectivity to butenes. U.S. Pat. No. 3,723,562 is hereby incorporated by reference.
CA-1,219,882 discloses a process for the production of 1-butene from C.sub.4 hydrocarbon mixtures containing 2-butene. The C.sub.4 hydrocarbon mixture is isomerized in a reaction zone to 1-butene in the presence of an acid catalyst. The isomerization zone effluent is distilled to recover a hydrogen offgas, a 1-butene side-draw product, and 2-butene in a bottom stream. The bottom stream is recycled to the isomerization zone for the further isomerization of the 2-butene to 1-butene. The process is operated in the absence of steam without significant cracking or skeletal isomerization.
International Patent Application No. 93/13013 to Kvisle et al. relates to an improved method for producing a silicon-alumino-phosphate catalyst which is more stable to deactivation by coking. The patent discloses that after a period of time, all such catalysts used to convert methanol to olefin (MTO) lose the active ability to convert methanol to hydrocarbons primarily because the microporous crystal structure is coked; that is, filled up with low volatility carbonaceous compounds which block the pore structure. The carbonaceous compounds can be removed by conventional methods such as combustion in air. In a paper by T. Inui titled "Structure-Reactivity Relationships in Methanol to Olefins Conversion on Various Microporous Crystalline Catalysts," which was included in Structure-Activity and Selectivity Relationships in Heterogeneous Catalysis, edited by R. K. Grasselli and A. W. Sleight, Elsevier Science Publishers B.V., Amsterdam, 1991, on pages 233-242, Inui discloses the highest reported ratio of ethylene/propylene produced from methanol over a SAPO-34 catalyst as about 15:1 (see FIG. 4, page 240).
Generally, the ratio of ethylene/propylene on a carbon basis varies from about 0.1 to about 10 and, more typically, varies from about 0.8 to about 2.5. Furthermore, ethylene and propylene yields are reduced by the production of heavier hydrocarbons such as C.sub.4 and C.sub.5 olefins. This narrow band limits the flexibility of the process and the value of the net products produced. Methods are sought to alter the product distribution of the MTO process for making light olefins to provide processing flexibility and overcome the equilibrium limitations of aluminophosphate catalyst of the MTO process. Methods are sought to reduce the production of C.sub.4 and C.sub.5 olefins from the MTO process relative to the production of ethylene and propylene. These and other disadvantages of the prior art are overcome by the present invention, and a new improved process for conversion of oxygenates to hydrocarbons is provided.